1. Field of the Invention
This invention relates to pressure sensors for detecting the pressures on measurement fluids and is capable of utilization for a pressure sensor based on an electrostatic capacity method, which detects pressure by utilizing changes in capacitance between opposite electrodes.
2. Description of the Related Art
As the prior art pressure sensor for detecting pressure of a measurement fluid, i.e., either liquid or gas, there are those which are based on an electrostatic capacity method or a strain method. These pressure sensors are provided with various internal seals for sealing individual pressure sections with the measurement fluid introduced thereto from external atmosphere sections or sealing the individual pressure sections from one another in order to obtain accurate measurement of the measurement fluid pressure or protecting individual internal circuits or the like from intrusion of the measurement fluid.
FIG. 17 shows a first prior art example of pressure sensor 480 (see Japanese Patent Laid-Open Publication No. H3-72230).
The pressure sensor 480 is based on the electrostatic capacity method. It comprises a pressure sensing element 481, which detects the pressure of a measurement fluid as gauge pressure or pressure difference, and a casing 482 enclosing the element 481.
The pressure sensing element 481 includes a base 483, a diaphragm 484 spaced apart at a regular interval by a spacer and extending parallel to the base 483, and opposite electrodes 485A and 485B provided on the opposite surfaces of the base 483 and diaphragm 484.
The space between the base 483 and diaphragm 484 constitutes a reference pressure chamber 487, which is filled with a fluid (usually atmosphere) under a reference pressure P2 which is led through a pathage 486 penetrating the base 483 and led to the outside of the casing 482.
A pressure chamber 488 is formed on the side of the diaphragm 484 opposite the reference pressure chamber 487, and it is filled with measurement fluid under pressure P1 which is led through a pathage 489 penetrating the casing 482.
A circuit or the like including electrode terminals (not shown), is provided on the side of the back 490 of the base 483. A pressure detection signal is supplied from such circuit through a cable 491 to the outside.
Around the periphery of the pressure chamber 488, an O-ring 492 is provided as a seal member between the diaphragm 484 and the casing 482 to have the pressure chamber 488 gas-tight and water-tight and prevent pressure escapement, while also preventing intrusion of the measurement fluid into the circuit or the like provided on the side of the back 490 of the base 483.
In this pressure sensor 480 based on the electrostatic capacity method, the measurement fluid that is introduced through the pathage 489 into the pressure chamber 488 pushes with its pressure the diaphragm 484, thus causing the diaphragm 484 to be curved to change the space between the base 483 and the diaphragm 484.
At this time, the inter-electrode distance between the opposite electrodes 484A and 485B is also changed to change the capacitance. The pressure of the measurement fluid thus can be detected by determining the change in the capacitance.
FIG. 18 shows a second prior art example of pressure sensor 500 (see Japanese Patent Laid-Open Publication No. S58-731).
This pressure sensor 500, like the first prior art example noted above, is based on the electrostatic capacity method, and it has a pressure sensing element 501 for detecting the pressure of a measurement fluid as absolute pressure.
The pressure sensing element 501, like the above first prior art example, has a base 503, a diaphragm 504 spaced apart at a regular interval by a spacer from and extending parallel to the base 503, and opposite electrodes 505A and 505B provided on the opposite surfaces of the base 503 and diaphragm 504.
The pressure sensing element 501 is covered by an aluminum case 508 via O-rings 506 and 507 as seal members, and the aluminum case 508 is covered by a casing 502. The pressure sensing element 501 and O-rings 506 and 507 are secured in position by mechanically caulking an end 511 o the aluminum case 508.
In the casing 502 and on the side of the back (i.e., on the upper side in FIG. 18) of the pressure sensing element 501, a circuit unit 509 is provided, which converts the electrostatic capacity between the opposite electrodes 505A and 505B into an electric signal. The circuit unit 509 is connected to the opposite electrodes 505A and 505B via leads 510.
The space 512 between the base 503 and the diaphragm 504, unlike the above first prior art example, is usually in vacuum.
On the side of the diaphragm 504 opposite the space 512, a pressure chamber 513 is defined by the diaphragm 504, aluminum case 508 and O-ring 506. To this pressure chamber 513 is led a measurement fluid under pressure measurement through a pathage 514 which is integral with the bottom of the aluminum case 508.
In this pressure sensor 500 based on the electrostatic capacity method, the measurement fluid that has been introduced through the pathage 514 into the pressure chamber 513 pushes the diaphragm 504 as pressure-receiving member with it pressure, thus causing the diaphragm 514 to be curved to change the space between the base 503 and the diaphragm 504.
At this time, the inter-electrode distance between the opposite electrodes 505A and 505B is also changed to change the capacitance. The pressure on the measurement fluid thus can be detected as absolute pressure by determining the change in the electrostatic capacity.
However, the seal provided by the O-ring 492 in the first prior art example of FIG. 17, is liable to become imperfect due to a positional deviation that might be produced when assembling the O-ring 492 in the casing 482 or to degradation of the O-ring 492.
In such a case, the pressure on the measurement fluid can no longer be detected accurately because of leakage of pressure in the pressure chamber 488. Besides, it is prone that the measurement fluid introduces into the circuit or the like on the side of the back 490 of the base 483, thus causing measurement errors. Further, an increase in the pressure in the Pressure chamber 488 may cause flexing of the diaphragm 484 toward the base 483 such as to cause degradation of the seal by the O-ring 492. Therefore, this example is not structurally excellent.
Further, the pressure sensing element 481 has its periphery in contact with and secured in position by the hard casing 482. When an external force is exerted to the casing 481, therefore, it will cause straining of the pressure sensing element 481 to cause an error in the measurement.
Further, in such case as when measuring the pressure difference, the pressure in the reference pressure chamber 487 and the pathage 486 has to be isolated from the external pressure. In such case, it is necessary to provide separate seal members from the O-ring 492 have to be provided in portion A or the like. This means that the number of components is increased, thus requiring increased man-hours for the assembling and leading to a cost increase. Further, this is undesired in view of the management of components.
With the seal by the O-rings 506 and 507 in the second prior art example shown in FIG. 18, unlike the first prior art example shown in FIG. 17, the pressure sensing element 501 is secured to the casing 502 via the O-rings 506 and 507 and aluminum case 508. Thus, when an external force is exerted to the casing 502, its effects are absorbed by the O-rings 506 and 507. That is, there is no problem of error generation in the measurement due to the straining of the pressure sensing element 501. Further, since the O-rings are provided on the opposite sides of the pressure sensing element 501, an excellent seal can be ensured.
However, since the pressure sensor 500 in the second prior art example includes the aluminum case 508, it is structurally complicated and has a large number of components, thus dictating increased man-hours for the assembling.
Further, the provision of the aluminum case 508 leads to a corresponding size increase of the pressure sensor 500.
Further, while the O-rings 506 and 507 and pressure sensing element 501 are secured to one another by mechanically caulking the end 511 of the aluminum case 508, in this case the O-rings 506 and 507 and pressure sensing element 501 are liable to be deviated in position relative to one another, thus resulting in an imperfect seal.
FIG. 19 shows a third prior art example of pressure sensor 900 based on the capacitance measurement (see Japanese Patent Laid-Open Publication No. H2-189435).
This pressure sensor 900 has a cylindrical casing 901 with a bottom. A pressure sensing element 903 is mounted in the casing 901 via an O-ring 902. The pressure sensing element 903 includes a base 904 having a large thickness and a diaphragm 905 having a small thickness and capable of deformation by the pressure of measurement fluid. The base 904 and the diaphragm 905 are spaced apart at a regular interval via a joining area 906 such that they extend parallel to each other. The opposite surfaces of the base 904 and the diaphragm 905 are provided with respective opposite electrodes 907 and 908, which form a capacitor 909. The pressure of the measurement fluid can be detected from a change in the capacitance of the capacitor 909.
A space 910 is defined between the base 904 and the diaphragm 905. The space 910 is held in vacuum. Another space 911 is formed on the side of the diaphragm 905 opposite the space 910. The measurement fluid is introduced into the space 911 from a port 912.
On the side of the back of the base 904, a space 914 is formed by an electric, connector 913 which is disposed such as to close the opening of the casing 901. This space 914 is tight sealed by sealing members 915 and 916 of rubber or like material.
In the space 914 a flexed base 917 is provided, and a measure circuit 918 is provided on the inner surface of the base 917. The measure circuit 918 is connected to opposite electrodes 907 and 908 to measure a change in the electrostatic capacity of the capacitor 109. The measure circuit 918 is connected to connector terminals 919 for such purposes as power supply, grounding, taking out output signal, etc.
FIG. 20 shows the measure circuit 918. In the Figure, the base 917 is shown in a developed state. The measure circuit 918 has an integrated circuit unit 920, a resistor 921, capacitors 922 and circuit patches which connect these circuit elements to one another.
In such third prior art example of the pressure sensor 900, the space 910 is held in vacuum, and a measurement fluid is introduced through the port 912 into the space 911 so that it acts on a pressured face 920 to cause flexing of the diaphragm 905. With the flexing of the diaphragm 905 the space between the electrodes 907 and 908, is changed. Thus, the pressure on the measurement fluid is detected as absolute pressure by making use of the change in the capacitance of the capacitor 909. Further, with this pressure sensor based on the electrostatic capacity method, the space (910 may be held under atmospheric pressure for gauge pressure measurement.
With the above third prior art example of the pressure sensor 900, however, in case when the insulation resistance of the surface of the base 917 changes due to a change in the relative humidity in the space 914, a leakage current is generated between connected portions of the opposite electrodes 907 and 908 in the measure circuit 918. In such a case, accurate pressure measurement can no longer be obtained. To avoid adverse effects of the ambient changes in the external atmosphere on the measure circuit 918, various measures have been provided.
More specifically, the space 914 is tight sealed and isolated from the external atmosphere to prevent adverse effects of the ambient changes in the external atmosphere. This means that the provision of the sealing members 915 and 916 for the tight sealing increases the number of components and complicates the structure. Besides, the process of manufacture is complicated, and the cost is increased. Further, a space for the tight sealing is necessary, thus increasing the size of the pressure sensor.
Further, such tight seal structure is liable to become imperfect due to degradation of the sealing members 915 and 916 and positional deviations during manufacture. To prevent this, a considerably complicated operation is required for obtaining the seal. In addition, it is necessasry to provide a sufficient amount of seal material.
Further, it is necessary to provide a capacitor, which can provide for a large capacity change due to the pressure from the measurement fluid in order that any leakage current generated due to a relative humidity change as noted above will have negligible effects. This, however, is undesired for the purpose of the size reduction of the pressure sensor.
Further, since the base 917 for mounting various components of the measure circuit 918 is provided in the space 914, it is necessary to secure a space for disposing the base 917, thus increasing the size of the pressure sensor.
Further, since the measure circuit 918 comprises a large number of components such as the integrated circuit unit 920, resistor 921, capacitors 922, etc., manufacturing process complication and manufacturing cost increase are dictated by the steps of disposing the large number of components in addition to the the step of installing the base 917.
Further, since the measure circuit 918 comprises a large number of components, contact failure is liable during manufacture, thus causing reduction of the product yield.
FIG. 21 shows a fourth prior art example of pressure sensor 800 (see Japanese Utility Model Laid-Open Publication No. H57-105943).
This pressure sensor 800, like the above third prior art example of the pressure sensor 900, comprises a base 801 having a large thickness and a diaphragm 802 having a small thickness and capable of being deformed with the measurement fluid pressure. The base 801 and the diaphragm 802 are spaced apart at a regular interval via a joining area 803 such that they extend parallel to each other. The opposite surfaces of the base 801 and the diaphragm 802 are provided with opposite electrodes 804 and 805, which form a capacitor 806. The pressure on the measurement fluid can be detected from a change in the electrostatic capacitance of the capacitor 806.
With the above fourth prior art example of the pressure sensor 800, unlike the third prior art example, there is no need of securing the space for installing the base 917. However, the measure circuit 807 includes many components such as the chip 810 and thin film resistor 811. Therefore, like the third prior art example the manufacturing process is complicated, and the manufacturing cost is increased. Further, because of the large number of components involved, it is difficult to reduce the size of the pressure sensor.
An object of the invention is to provide a pressure sensor, which permits accurate measurement of pressure while being ready to manufacture, permitting size reduction and being excellently durable.